Assail is an action word that means to physically attack someone or something, either verbally or physically. Synonyms for assail include assault, attack, jump, pounce, rush, strike, storm, and invade. Other possible synonyms are beset, belabor, besiege, harass, molest, torment, plague, and badger. If we want to go a bit more literary, we could use words like impugn, vituperate, calumniate, denounce, or castigate. In short, there are many alternatives to the word assail that can be used depending on the context, tone, and level of aggression that we want to convey in our writing.